Well-designed combination therapies can maintain or increase the potency of the antitumor effect while abrogating toxicity. We have shown that the combination of PDT with TNF-a provides outstanding therapeutic efficacy without increasing toxicity. Yet, this combination may not be clinically viable since systemic TNF-a can have significant toxicity in patients. In the previous funding period we confirmed our original hypothesis that the vascular targeting agent DMXAA would strongly enhance the antitumor activity of PDT while maintaining treatment selectivity. This hypothesis was based on the earlier observation that DMXAA selectively induces TNF-a in tumor tissue. Recent studies suggest that TNF-a induction may not be entirely responsible for the therapeutic synergism between PDT and DMXAA, suggesting other compensatory mechanism(s). It has been shown that the antivascular activity of DMXAA can also result from direct effects on the tumor vascular endothelium and indirect effects of other DMXAA-induced cytokines/mediators besides TNF-a. Therefore, we now plan to closely define the roles of TNF-a and direct endothelial cell apoptosis, as well as that of 5-HT/serotonin, NO and VEGF, in the enhancement of PDT by DMXAA;we hypothesize that by understanding the underlying mechanism of the interaction between DMXAA and PDT regimens will be able to identify a biomarker or surrogate marker that will enable us to develop optimal preclinical and clinical treatment designs. The mechanisms that govern the response to PDT are complex and not well understood;it has recently become clear that the choice of PDT regimen, in particular fluence rate, governs the pattern of tissue response (i.e. the relative contributions of vessel damage and direct tumor cytotoxicity, as well as the strength of the induction of innate and adaptive immune responses) as well as treatment outcome. We have described this in an experimental tumor model using the 2nd-generation photodynamic sensitizer HPPH. We believe that the degree and mechanism of interaction between DMXAA and HPPH-PDT will strongly depend on the PDT treatment regimen. We hypothesize that a PDT regimen will be identified that, in combination with DMXAA, achieves high antitumor efficacy, high selectivity and clinically practicable treatment times. Our group has demonstrated that PDT stimulates innate and adaptive immune responses resulting in increased tumor immunogenicity;we have now shown that DMXAA enhances this response;we now hypothesize that low doses of DMXAA can be used to increase the ability of PDT to augment antitumor immune memory and thus provide control of tumors outside the treatment field. Our Specific Aims are to: 1) Characterize the underlying mechanism(s) of interaction for the enhancement of HPPH-PDT by DMXAA;2) Determine the mechanism by which DMXAA augments the ability of PDT to enhance antitumor immune memory;3) to bring the preclinical research of the past funding period to clinical fruition we will examine the activity of the combination of DMXAA and Photofrin-PDT against basal cell carcinomas (BCCs).